Fixed a bug in the initialization of TIM6 used for spektrum binding.

The roason was that the prescaler value was not being loaded until an
update event was generated. Remedied setting the counter value so that
the update get's generated virtually instantly after the timer gets
enabled.  Cleaner solution would be to force an update event by setting
some bit, but I don't seem to see anything that would do the job.

sw/airborne/arch/stm32/subsystems/radio_control/spektrum_arch.c

@@ -720,12 +720,23 @@ static void SpektrumDelayInit( void ) {
   /* Enable timer clock */
   rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM6EN);
 
+  /* Make sure the timer is reset to default values. */
+  timer_reset(TIM6);
+
   /* Time base configuration */
-  timer_set_mode(TIM6, TIM_CR1_CKD_CK_INT,
-	         TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
+  /* Mode does not need to be set as the default reset values are ok. */
   timer_set_period(TIM6, UINT16_MAX);
   timer_set_prescaler(TIM6, (AHB_CLK / DELAY_TIM_FREQUENCY) - 1);
 
+  /*
+   * Let's start the timer late in the cycle to force an update event before
+   * we start using this timer for generating delays. Otherwise the prescaler
+   * value does not seem to be taken over by the timer, resulting in way too
+   * high counting frequency. There does not seem to be a force update bit on
+   * TIM6 is there?
+   */
+  TIM6_CNT = 65534;
+
   /* Enable counter */
   timer_enable_counter(TIM6);
 }
@@ -733,12 +744,14 @@ static void SpektrumDelayInit( void ) {
 /* wait busy loop, microseconds */
 static void DelayUs( uint16_t uSecs ) {
   uint16_t start = TIM6_CNT;
+
   /* use 16 bit count wrap around */
-  while((uint16_t)(TIM6_CNT - start) <= uSecs);
+  while((TIM6_CNT - start) <= uSecs);
 }
 
 /* wait busy loop, milliseconds */
 static void DelayMs( uint16_t mSecs ) {
+
   for(int i = 0; i < mSecs; i++) {
     DelayUs(DELAY_TIM_FREQUENCY / 1000);
   }